Refuse derived fuel (RDF) is a product of solid waste processing plants in which refuse such as municipal solid waste or industrial solid waste is treated in order to substantially reduce the amount of waste which is actually applied to landfills. The heat value of refuse derived fuel typically is about one-half to perhaps two-thirds that of anthracite coal and it can be used as a sole or primary fuel for power plants and the like or as a co-fuel in which it is mixed with another hydrocarbon fuel, typically coal.
It is a conventional practice to produce refuse derived fuel in the form of compacted pellets which are stored until needed as fuel in a combustion process. For example, U.S. Pat. No. 4,561,860 to Gulley et al. discloses a process in which raw waste is passed through a pulverizing and screening system to remove fines and large materials to produce an intermediate product which is supplied to an air classifier. The air classifier separates relatively light products such as paper and plastics from heavier products such as metal and glass. The light product is applied through a shredder and densifier to a drier where its moisture content is reduced to about 17%. The drier output is optionally mixed with coal dust in an amount of about 25 to 50 wt. %. The mixture of coal and refuse material is supplied to a densifying and pelletizing system in which the mixture is fed to a roller mill and extruded through a die screen to form the densified RDF pellets.
Binders may be incorporated into the refuse derived fuel in order to arrive at RDF pellets of enhanced compressive strength. For example, U.S. Pat. No. 4,405,331 to Blaustein et al. discloses refuse derived fuel pellets which incorporate a binder-fly ash system in addition to coal fines which may be optionally added to increase the heating value of the fuel. The preferred refuse component in Blaustein et al. is derived from solid refuse consisting substantially of solid cellulose based material such as paper and cardboard, plastic material such as styrofoam and soft plastics, soft metal wastes such as paper clips, staples and the like as well as glass. Preferably, the refuse component used in the Blaustein et al. process is substantially free of glass and metallic materials. Suitable binders disclosed in Blaustein include cornstarch, portland cement, asphalt emulsion and lignin. The RDF is extruded in pellets in the shape of cylinders 3/4" in diameter by about 3/4" in length having a dry form compressive stress of about 30 psi and a dry form moisture content of less than about 10% by weight. The RDF pellets produced by the Blaustein process have a heat value in excess of about 6000 BTUs per pound, permitting its use either as a primary or as a supplemental fuel source.
As noted previously, refuse derived fuel may be used as a sole fuel source or it may be used as a co-fuel with a fossil fuel such as coal. An advantage of refuse derived fuel is that it usually is of relatively low sulfur content, containing usually less than one-quarter percent but no more than one-half percent sulfur. Coal, on the other hand, is often relatively high in sulfur content, and as a consequence, steps must be taken when burning coal to reduce sulfur emissions. For example, as disclosed in U.S. Pat. No. 4,230,460 to Maust, a mixture of finely divided lime, limestone, or dolomite and flyash is added to powdered coal to provide a binder enabling the coal to be formed into pellets or briquets. The Maust process is proposed as an alternative to a fluidized combustion process in which crushed coal is burned in the presence of an agent such as limestone or dolomite to convert sulfur dioxide to solid calcium sulfite sulfate containing reaction products. The lime is described as a binder constituent which functions as a binder itself and as an initiator for a pozzolanic cementing action and as a desulfurizing agent during combustion. The lime and flyash are present in the form of a very finely divided powder which promotes a high degree of dispersion throughout the powdered coal to maximize the cementing action. The cement-like binder formed in the Maust process is said to break down upon combustion, regenerating lime and flyash in the fire box. The finely divided lime would then be available for in-furnace desulfurization of combustion gases with both reacted and unreacted lime particles being swept out of the fire box as entrained particulates.
Another procedure for upgrading coal is disclosed in U.S. Pat. No. 4,758,244 to Harvey et al. In the Harvey process, densified coal pellets of residual water content and enhanced calorific value are produced from brown coals which are comminuted by shearing to produce a wet plastic mass which is extruded into pellets. Sodium carbonate is added to the aqueous phase during plasticization in order to increase the strength of the densified product. Alkaline earth metal carbonates may also be employed and in addition, magnesium and calcium hydroxides can be added to provide a two-fold action, increasing the pH of acid coals while forming electrostatic bridge bonds utilizing acidic groups on the adjacent coal particles. In a specific example, 5% magnesium hydroxide produced an average compressive strength of 61 MPa as contrasted with 11 MPa for pellets without additives, while calcium hydroxide in amounts of 2% and 5% produced compressive strengths of 39 and 65 MPa, respectively.